Backpage.com, an online classified advertising service, was sued by two women who claim the website operator helps pimps and traffickers in the sexual exploitation of minors.

The company “profits substantially” from enabling illegal commercial sex transactions, including by shielding the identity of those who post ads and deceiving law enforcement about internal protocols for detecting sex ads involving children, the women, identified as Jane Doe 1 and Jane Doe 2, said in a complaint filed today in Boston federal court.

Users of Dallas-based Backpage.com, the second-largest online classified ad service in the U.S., can post ads for everything from motorcycles for sale to community events.

“They really are the dominant force in the online Internet sex trade today,” said John Montgomery, a lawyer at Ropes & Gray LLP, which filed the suit on a pro bono basis. “Our case here is not about the content of the ads, but rather the structure and purpose of a business that has become an essential participant, and really driver, of the market.”

Liz McDougall, a lawyer for Backpage.com, declined to comment on the lawsuit, saying she just received it and hadn’t had a chance to review it.

Sexual exploitation

In the past two years, Backpage.com has prevailed in barring enforcement of laws in Washington, Tennessee and New Jersey that aimed to hold online service providers criminally liable for advertisements that turned out to involve sexual exploitation of a minor, McDougall said.

By trying to hold the service liable for third-party information, the laws were found to violate the federal Communications Decency Act as well as the U.S. Constitution’s First Amendment right to free speech, according to McDougall.

In 2009, a federal judge in Chicago dismissed a lawsuit against Craigslist Inc. that accused the the classified ads website of creating a public nuisance by maintaining a forum used for prostitution services. The judge found that the site wasn’t legally liable for the content of ads published on the site by others.

The following year, amid pressure from state attorneys general, Craigslist removed a category for adult services, which is when Backpage.com undertook to become the dominant player in the illegal online sex market, according to the complaint filed today.

This latest suit differs from previous legal attempts to hold Internet classified ad providers accountable for facilitating commercial sex transactions, in that it accuses Backpage.com of “substantially contributing to the trafficking operations,” particularly with respect to minors, Montgomery said.

The website facilitates at least 80 percent of all online commercial sex advertising in the country, according to the lawsuit.